在光滑的水平面上方，有两个磁感应强度大小均为B，方向相反的水平匀强磁场，如图．PQ为两个磁场的边界，磁场范围足够大．一个边长为a、质量为m、电阻为R的金属正方形线框，以速度v垂直磁场方向从如图实线位置开始向右运动，当线框运动到分别有一半面积在两个磁场中时，线框的速度为 $\frac{v}{2}$ ，则下列说法正确的是（   ）

    Motto is a short statement giving a rule on how to behave , ____ can inspire us especially when we are ____ (face) with difficulties. Many of us have our mottoes, such as “Where there is a will, there is a way.” Or “Nothing is difficult if you put your heart into it.”, and so on. My motto is “God helps those who help____.” Sometimes I am lazy and don't want to make efforts ____ (work) hard, ____ the moment I think of my motto I will get energetic again and devote myself ____ what I am doing. I write my motto ____ I can see it easily. Every time I fail in my exam and begin to lose heart, the sight of my motto inspires me with much ____ (confident). My motto also makes me become ____ independent person. That is to say, I won't rely on others easily.

    My friends, what is your motto? It is really ____ (benefit) to you. So please choose one as soon as possible.

如图为人体某处的血管结构和血流方向示意图．下列有关叙述正确的是（  ）